8.3.1 Input Common-Mode Voltage Range

When the primary design goal is a linear amplifier circuit with high CMRR, it is important to not violate the input common-mode voltage range (V_ICR) of the op amp. The typical specifications for this device are 0.2 V below the negative rail and 1.1 V below the positive rail.

Assuming the op amp is in linear operation, the voltage difference between the input pins is small (ideally 0 V); and the input common-mode voltage is analyzed at either input pin with the other input pin assumed to be at the same potential. The voltage at V_IN+ is simple to evaluate. In a noninverting configuration, as shown in Figure 45, the input signal, V_IN, must not violate the V_ICR for this operation. In an inverting configuration, as shown in Figure 46, the reference voltage, V_REF, must be within the V_ICR. Assuming V_REF is within V_ICR, the amplifier is always in the linear operation range irrespective of the amplitude of the input signal V_IN.

The input voltage limits have fixed headroom to the power rails and track the power-supply voltages. For a 5-V supply, the linear input voltage ranges from –0.2 V to 3.9 V and –0.2 V to 1.6 V for a 2.7-V supply. The delta headroom from each power-supply rail is the same in either case: –0.2 V and 1.1 V.